The present invention relates to an active matrix liquid crystal display (AM-LCD) which employs thin film transistors (TFTs).
AM-LCDs are extensively used in electronic instruments, such as personal computers and the like. In an AM-LCD, thin film transistors are formed on a highly insulating substrate. This causes the substrate to be liable to be charged by static electricity, which can change the thin film transistors.
A first prior art AM-LCD 100 having an anti-electrostatic destruction structure will be described with reference to FIG. 1. The AM-LCD 100 comprises an amorphous silicon (a-Si) substrate 1, a plurality of picture element (pixel) cells 2 disposed in a plurality of rows and columns on the substrate 1, a plurality of gate control lines 6 and data lines 7 which are connected to the pixel cells 2, and a shorting ring wiring 8 connected to both of the lines 6 and 7. The plurality of pixel cells 2 form a pixel cell array. Each pixel cell 2 includes a TFT 3, a liquid crystal 4 and a capacitor 5.
The gate control lines 6 are disposed on the substrate 1 and extend along the rows of pixel cells 2 and the data lines 7 are disposed on the substrate 1 and extend along the columns of pixel cells 2. The TFT 3 has a drain connected to the liquid crystal 4 and the capacitor 5, a gate connected to the gate control line 6 and a source connected to the data line 7.
The shorting ring wiring 8 extends around the edges of the substrate 1, and is formed by substantially the same process used to form the gate control lines 6 and the data lines 7. In the latter process, the gate control lines 6 are formed before the data lines 7 are formed. After forming the pixel cells 2, the edges of the substrate 1 are severed along a cutting line P located inside the shorting ring wiring 8 prior to conducting a panel test and a TAB mounting step. The wiring 8 is thus severed from the gate control lines 6 and the data lines 7.
In the manufacturing step or steps which occur between forming and severing the shorting ring wiring 8, the gate control lines 6 and the data lines 7 are shorted together via the shorting ring wiring 8. Thus, if the substrate 1 is charged by static electricity, causing either the gate control line 6 or the data line 7 to assume a high potential, by antenna effect, any potential difference between the gate and the source of the TFT 3 is eliminated by the shorting ring wiring 8, which prevents electrostatic destruction of the pixel cell 2 between the gate and the source electrode from occurring.
A second prior art AM-LCD 110 using a polysilicon (p-Si) substrate 9 will now be described with reference to FIG. 2. The AM-LCD 110, in addition to the components of the AM-LCD 100, includes a gate control circuit 10 and a signal control circuit 11 disposed on a peripheral region of the polysilicon substrate 9.
The gate control line 6 has first end connected to the gate control circuit 10 and a second end connected to the shorting ring wiring 8. Similarly, the data line 7 has a first end connected to the signal control circuit 11 and a second end connected to the shorting ring wiring 8. As in the first example, the shorting ring wiring 8 is severed along the cutting line P and separated from the gate control lines 6 and the data lines 7. Again, the shorting ring wiring 8 acts to prevent electrostatic destruction of the pixel cell 2 between the gate and the source electrode from occurring.
However, electrostatic destruction is not satisfactorily prevented by the AM-LCD""s, 110 for the reasons mentioned below.
(1) The electrostatic destruction of the TFT 3 across the gate and the source electrode may be prevented, but a potential difference between the gate electrode and the drain electrode to which the liquid crystal 4 and the capacitor 5 are connected is not eliminated, and thus may cause electrostatic destruction across the drain and the gate electrode of the TFT 3.
(2) The provision of the shorting ring wiring 8 is effective only from the step of forming the gate control lines 6 and the data lines 7 to the step of severing the substrate 1, 9 along the cutting line P. However, the gate control lines 6 are formed by a wiring layer which is distinct from a wiring layer forming the data lines 7. Normally the gate control lines 6 are formed first, and then the data lines 7 are formed. Accordingly, static electricity may be generated on the substrate 1, 9 after the gate control lines 6 are formed, but before the data lines 7 are formed. If the antenna effect causes the gate control line 6 to assume a high potential, the electrostatic destruction of TFT 3 may occur between the gate electrode and either the source or the drain electrode.
(3) As substrate 1, 9 is severed along the cutting line P, the gate control lines 6 and the data lines 7 are exposed at the edges of the substrate 1, 9, and accordingly, it is possible for static electricity to reach the gate control lines 6 and the data lines 7 and cause electrostatic destruction of the TFT 3 during the step of mounting the substrate 1, 9. A substrate without a frame has recently been used in electronic instruments to achieve a reduction in the size and weight thereof. In these circumstances, the exposed ends of the gate control lines 6 or the data lines 7 provide an access port for static electricity.
(4) In the step of severing the shorting ring wiring 8, static electricity may be generated as the substrate 1, 9 is severed, and damage the TFT 3. The gate control lines 6 on the substrate 9 are connected to an output stage of the gate control circuit 10 and the data lines 7 are connected to an output stage of the signal control circuit 11. Accordingly, the output stage of either the gate control circuit 10 or the signal control circuit 11 may be subject to electrostatic destruction during the step of severing the shorting ring wiring 8.
(5) It has been proposed to sever the shorting ring wiring 8 on the substrate 1, 9 with a laser to prevent exposure of the gate control lines 6 and the data lines 7 at the edges of the substrate. It has also been proposed to seal the edges of the substrate 1, 9 with synthetic resin to provide an electrical insulation. However, the implementation of such steps requires a modification of the manufacturing process, increasing the manufacturing cost.
(6) It has also been proposed that the gate control lines 6 and the data line 7 be exposed at the edges of the substrate 1, 9 via interposed elements which serve as resistors for the static electricity. However, these elements are not effective in preventing electrostatic destruction in a satisfactory manner.
It is an object of the invention to provide a liquid crystal display which has a high reliability in guarding against the static electricity.
In one aspect of the present invention, a liquid crystal display includes a substrate, a pixel cell array disposed on the substrate, a plurality of gate control lines, a plurality of data lines, and a termination unit. The pixel cell array includes a plurality of pixel TFTs arranged in rows and columns. Each of the gate control lines has a first end and a second end, and extends along a respective row of the pixel cell array and is connected to the pixel TFTs of that row. Each of the data lines extends along a respective column of the pixel cell array and is connected to the pixel TFTs of that column. The termination unit is connected to the pixel TFTs by way of the gate control lines. The termination unit discharges static electricity from the gate control lines such that damage to the pixel TFTs caused by static electricity is prevented.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.